An example of a known application for white, biaxially oriented polyester films is lids for yoghurt cups. In that application they are used as an alternative to aluminum foils. These lids are printed and embossed on the outer side and on the inner side are provided with a coating or hot-melt, which adhesive-bonds the lids to the cups. To improve the shelf life of the yoghurt, sufficient protection from light has to be provided not only by the cup but also by the lid. This is generally achieved via coloring of the lids with suitable pigments. For specific applications, it is also desirable for the complete lid or the complete system to be sterilizable, but at least steam-sterilizable. Another requirement placed upon the lid film is that when it is peeled from the cup it is subject neither to delamination nor tearing nor tear-propagation. The properties required for this are present in the film by virtue of the biaxial stretching process and by virtue of appropriately high thickness. The mechanical properties of the lid (stiffness, puncture resistance, tear-propagation resistance) improve more than proportionally with the thickness of the film. Polyester films of thickness from 30 to 90 μm have proven successful for the yoghurt-lid application. The thickness of the film cannot be increased without limit, because materials costs rise with thickness and sealing cycle times deteriorate, and this is undesirable for economic reasons. A trend toward lower thicknesses is observable.
Lids comprised of polyester films generally have a number of advantages:                The lid has a smooth, high-gloss surface which guarantees excellent color reproduction and therefore has high visual appeal.        When compared with conventional material, the puncture resistance of the film is higher by up to 700%, and this gives substantially greater product protection.        The lid is easy to open without retention of lid fragments on the cup, as is found with conventional materials, e.g. aluminum foils. This property is particularly valued by the consumer.        Complete separation of cup and lid is highly advantageous for recyclability of the two materials. The lid film is comprised of thermoplastic polyester which has excellent recycling properties. The product is therefore environmentally friendly.        The polyester film moreover gives excellent processing for the lid application because it has high stiffness and good slip.        The lid is moreover absolutely metal-free. This guarantees a very high level of safety during metal detection, the use of which is constantly increasing in food production. The proportion of any metallic constituents which may be present in the contents can therefore be determined with high precision using that method.        
Attempts to equip a conventional transparent film with the desired combination of properties via pigment addition has failed. The film delaminated on peeling from the yoghurt cup.
Processors use certain criteria for the performance of the films during further processing to give yoghurt cups. The test variables usually used in the production of lid films are the R value and the emax ratio. The R value can be given as a measure of orientation and is measured directly after production of the film in the middle of the film web. The emax value ratio describes the distribution of orientation across the width of the web (known as bow) and is likewise measured during production of the film, but at discrete intervals across the entire width of the film web (cf. under test methods). In the case of transparent films (not the present white films), the R values are in the range from 45 to 48 and the emax ratio values are in the range from 2.6 to 2.8. Studies have shown that these values cannot be extended from transparent films to the present white films. If the abovementioned values are maintained for white film, the result is generally delamination of the film. The film delaminates within itself, and tears in the direction of the thickness; the tear damages the structure of the film and propagates within the film.
The properties mentioned above are not achieved in their entirety by films known in the prior art.
For example, EP-A-0 605 130 describes a multilayer film for the lid application which has at least one opaque layer and at least one transparent layer comprised of crystalline polyester. The film is also described via a deformation index, which is to be greater than or equal to 2.5%. In addition, the film can be coated with certain substances, improving adhesion to printing inks and/or other inks. The specification gives no information concerning the R values and the emax ratio of the film. A film produced to EP-A-0 605 130 (Example 1, longitudinal stretching temperature about 80° C., stretching ratio for longitudinal stretching 3.3) delaminated, and moreover exhibited poor processing performance because the film curled after stamping-out to give the lids.
For economic reasons it is necessary to achieve a permanent reduction in the costs for production of the lid film. A significant lever here is the thickness of the film. Lower lid thickness is directly associated with a reduction in materials costs and leads to higher filling speeds, due to low sealing cycle times. However, the thickness of the lid cannot be reduced as desired, because that leads to problems in processing and in handling. Lids comprised of polyester film with excessively low thickness tend to tear and to delaminate. The result is loss of product safety and of the advantage of 100% separation between lid and cup. The more pigments, and the greater the size of the pigments, incorporated into the polyester film, the greater the risk of tearing and of delamination of the lid. Pigments, in particular relatively coarse pigments, form points of weakness within the film at which breakdown of the film begins on peeling from the cup.
It is therefore an object of the invention to provide a white, biaxially oriented polyester film, in particular for the lid application, which has better properties than the polyester films established within the market, and does not have the disadvantages of the prior art.